Defence Secretary Geoff Hoon said the crash was not the result of enemy action.

Prime Minister Tony Blair said Britain owed an "immense debt of gratitude" to the troops who had died.

"These were brave men who in order to make us safer and more secure faced the risks and had the courage to serve the country and the wider world," he said.

It is thought there were problems from the start of Mr Evans' mission as pilots complained of the thick, black smoke from oil-filled trench which the Iraqis had set alight.

The first helicopter in the mission was forced to return, but the pilot of the fated machine decided to press on.

The aircraft was seen to nosedive into the ground eight miles south of Basra.

One of those in following helicopters was Lieutenant Commander Jon Pentreath.

He said: "That was a very sobering moment.

"We did not know what it was initially - we thought maybe it was a missile coming in but we found out 20 minutes, half an hour later, it was an aircraft which had crashed."

An investigation into the cause of the crash, which also killed four US servicemen, has been promised by defence chiefs.

Risk

A former US Airforce pilot told the BBC he thought the crash was probably caused by the age of the aircraft, which had been service for almost 40 years.

But the scale of the US-UK operation is bound to create a risk to troops, according to one senior officer.

Group Captain Jon Fynes was based at RAF Valley on Anglesey for eight years - where he oversaw pilots' low-level flight training - before being deployed to the Gulf.

"In an environment such as this, with so many helicopters and aircraft operating, there is always going to be a risk."

'Support'

Colonel Philip Wilkinson, formerly of 3 Commando Brigade, said: "Procedures would have been put in place to ensure that the families of the bereaved will get maximum support in every sense, emotional and financial from the system.

"And the system will be the family of 3 Commando Brigade."

A Royal Navy spokeswoman said they were busy handling calls from anxious families following the crash.

The Royal Marine's death comes as Welsh troops have been spearheading the military advance in Iraq and have also started building camps to hold hundreds of prisoners-of-war.